This invention relates to a fuel assembly of an improved construction for a nuclear reactor.
The core of a boiling-water reactor is generally located in a housing within a pressure vessel. The core is constructed such that a coolant can flow in the direction of the axis of the housing, with the core being generally immersed in a pool of coolant during normal operation of the reactor.
During normal operation of a boiling-water reactor, the coolant is caused to circulate in the pressure vessel by a coolant circulating device.
A spray cooler is provided in an upper space of the housing by way of precaution against an accident involving leaks of the coolant from the pressure vessel due to a failure of the coolant circulating device. In the event that such accident occurs, a coolant is sprayed on to the entire surface of the core. The pressure vessel is enclosed in an airtight sealed dry well.
The prior art patents include U.S. Pat. Nos. 3,350,275 and 3,338,991 wherein the detailed construction of the core or the fuel assemblies is described.
In the event that an accident involving a failure of the coolant circulating device occurs, problems described hereinafter will arise:
(1) Upon occurrence of such accident, the coolant in which the core is immersed will leak out into the dry well in several scores of seconds, and spraying of the coolant by the spray cooler will be initiated. The coolant which is sprayed will be heated and vaporized by the decay heat of the fuel in the core and the vapor will flow upwardly, thereby suppressing the inflow of additional coolant into the core and reducing the quantity of the coolant flowing downwardly through the core.
(2) Thus, the coolant sprayed will pass through passages other than that of the core which offer greater resistance to its flow than the core, before being collected in the bottom of the pressure vessel. Accordingly, it takes time for the core to be immersed in a pool of coolant again.
(3) The nuclear fuel is encased in a clad tube made of a zirconium alloy. However, if there is any delay in cooling the clad tube by bringing the same into contact with a sprayed coolant as described in paragraphs (1) and (2), there will be the hazard of the clad tube losing its heat resisting strength because of its temperature exceeding 800.degree. C. and reaching the embrittling temperature of 1200.degree. C. due to decay heat.
Also, during normal operation of a nuclear reactor, the following problem will be encountered:
(1) There is a difference in the resistance offered to the flow of the coolant between the central portion of the core and the peripheral portion thereof. Particularly, when it is necessary to locally change the number of fuel elements contained in the fuel assemblies, the flow rate of the coolant flowing through the core will become unbalanced from portion to portion.
(2) As a result, the power distribution in the core will become non-uniform.